Buck Converter Primer and Reference I

The most basic and ubiquitous topology of any power distribution network is the buck converter since in embedded systems you usually want to downconvert a USB or Li-Ion battery voltage to supply a microcontroller. See Wikipedia or one of the hundreds of app notes on the subject for the full derivation, but it basically works by switching on and off the output at some percentage to scale down the input voltage.

Suppose for instance you wanted the output voltage (Vout) to be half of the input voltage (Vin). The converter would turn the output on and off 50% of the time for every cycle. During the remainder of the time, the energy stored in the inductor and capacitor keeps the output at the correct voltage until the cycle repeats. By selecting the correct feedback resistors R1 and R2, the converter will automatically switch the output at the appropriate ratio to get the desired output voltage.

Here is a standard implementation of a buck converter. The block contains power switches and the control circuitry to make the switches switch at the right time.

Usually the vendor of the power converter IC will tell you in the datasheet exactly what supporting components to use and there are only a few things you will need to calculate.

This is the relationship between Vout, R1, R2, and Vref, which is given in the datasheet:

Here are some useful spreadsheets to help design a buck converter circuit:







 When selecting the feedback resistors, make sure the current they draw from the output is much greater than the current drawn by the fb pin in order to make the output as accurate as possible. This spreadsheet snippet helps to solve for this:

Standard Resistor Value Calculator

Type in a number and the spreadsheet will return the closest resistor value from the E96 and E24 series. I found the formulae on the Electronic Design magazine website (http://electronicdesign.com/components/excel-formula-calculates-standard-1-resistor-values) and it is too useful not to want available everywhere.

Digital Level PCB

The idea for a digital level in this particular form-factor occurred to me while lifting weights in the gym. I notice my gym buddies aren't so great at keeping the barbell level during their bench presses and maybe mine aren't much better. Why not throw a bunch of electronics at the problem?

This is a microcontroller-based digital level utilizing a 3-axis accelerometer to measure inclination. I added seven discrete LEDs to serve as a visual indication of tilt and four buttons for as yet to be determined functionality. Perhaps to 'zero' the level and change operating mode and settings.

Besides the digital level functionality, I am going to implement a countdown timer mode, and conceivably in the future maybe a rep-counter.

The overall board ended up 5x1", and could attach to the barbell with cable ties or velcro through the mounting holes. The system is powered by a CR2032 coin cell contained on the board which should permit many hours of continuous use.

Analog Accelerometer Amplifier and Filter Circuit

Circuit designs I do at home and post on this site will generally be simple enough to implement on a two-layer PCB and exclusively utilize hand-solderable components. This is so I have a chance to actually build and test my circuits on a reasonable budget without a professional rework station, and also because I am using a free and somewhat limited CAD tool.

This is a single-axis analog output accelerometer and signal conditioning circuit. The Analog Devices ADXL001 accelerometer has very wide bandwidth for a MEMS accelerometer, and my intent is to use it in an audio application. I have no idea what the chances of this working are, but I will follow up once I have a prototype in-hand and operational.

As I do not really have any idea what to expect the output levels to be, I made the signal conditioning chain as versatile as possible; R1 and R2 allow me to scale the output, R1 and C8 form a RC low-pass filter, C3 and R3 form a high-pass filter and AC-couple the signal, etc. I chose the opamp to optimize power and noise, and enough GBW to cover the audio band.

The circuit was small enough to finish the layout in only a couple hours. DesignSpark PCB is really great for circuits of this scope because it has a very extensive library of components and is fairly intuitive.

Thanks to OSH Park batch PCB service, I'll be getting 6 board in about 2 weeks for just $17!

Click to View Schematic Full Size

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I am an electronic design engineer by day, and am currently devoting additional avocation-cycles to this pursuit as well. I have created this page in order to aggregate circuits, projects, formulae, or other interesting content of a technical nature.
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-SL